(a) Field of the Invention
The invention relates to a bottom emission type electroluminescent display.
(b) Description of the Related Art
Electroluminescent displays are active light-emitting display devices and have many advantages over cathode ray tubes (CRTs) or liquid crystal displays (LCDs). For example, electroluminescent displays have a wide viewing angle, high contrast, fast response speed, light weight, small size, thin thickness, and low power consumption. Therefore, there has been an increasing interest in electroluminescent displays. Electroluminescent displays are generally classified as inorganic electroluminescent displays and organic electroluminescent displays based on whether a light emission layer is made of an inorganic material or an organic material.
FIG. 1 shows a conventional electroluminescent display having a structure similar to the structure disclosed in U.S. Pat. No. 4,539,507. The electroluminescent display of FIG. 1 has a stacked structure including an anode 20, a medium layer 30, and a cathode 40 on a transparent substrate 10. As shown in FIG. 1, the medium layer comprises a hole injection layer 31, a hole transport layer 32, a light emitting layer 33, an electron transport layer 34, and an electron injection layer 35. However, all the layers, except for the light emission layer, may be omitted as needed. The anode 20 is made of a transparent material, such as indium tin oxide (ITO). While some light emitted from the light emitting layer 33 is reflected by the cathode 40 and then discharged through the anode 20 and the transparent substrate 10, other light is directly discharged through the anode 20 and the transparent substrate 10 without being reflected by the cathode 40. However, since light emitted from the light emitting layer undergoes once constructive interference one time before being discharged from the electroluminescent display, enhancement of light emission characteristics is slight.
FIG. 2 shows a top emission type electroluminescent display as disclosed in Applied Physics Letter, Vol. 63, 594 (1993). In this electroluminescent display, some light emitted from a light emitting layer 33 undergoes constructive interference by anode 20 and a partial reflection layer 50.